U.S. Pat. No. 6,359,276 discloses an infrared sensor composed of a thermal infrared sensing element and a semiconductor device which are arranged in a side-by-side relation on top of a semiconductor substrate. The thermal infrared sensing element is held on a sensor mount which is formed in the top surface of the substrate as being supported by means of beams extending from the sensor mount to a remainder of the substrate. The sensor mount and the beams are defined by a portion of the top surface of the substrate which is converted into a porous structure for thermal insulation of the infrared sensing element from the remainder of the substrate. That is, the sensor mount and the beams are formed by anodizing a top part of a doped region formed in the surface of the semiconductor substrate into a porous body. Thus, the prior art makes the best use of the semiconductor substrate to realize the sensor mount in the top surface of the substrate. However, since the sensor mount is limited to within the top surface of the substrate, it is practically impossible to form the semiconductor device in the semiconductor substrate immediately below the sensor mount or the senor element. More specifically, since the porous beams extend only within the top surface of the substrate, the sensor mount cannot be lifted above a general top plane of the substrate.
Japanese patent publication No. 2000-97765 discloses another prior art in which a sensor mount is supported in a spaced relation from a substrate to mount an infrared sensing element for making sufficient thermal insulation of the sensing element from the substrate. In this instance, the sensor mount is supported by means of beams which slant downwardly and outwardly from the sensor mount and terminate on the substrate. The beams as well as the sensor mount are made of a silicon oxide or silicon nitride which is assumed to give a sufficient mechanical strength for supporting the sensor mount and the infrared sensing element in a spaced relation from the top surface of the substrate. Nevertheless, such supporting structure using the slanting beams is found not suitable for maintaining the infrared sensing element accurately at a desired height. The accurate positioning with respect to the height is particularly important when a plurality of the sensing elements is arranged in a two-dimensional array. When the beams and the sensor mount are required to be made of a porous material for enhanced thermal insulation as in the above U.S. patent, however, the supporting structure of the sensor mount is not sufficient for stably supporting the infrared sensor with the use of the slanting beams, and therefore requires a specific design which is not derived from either of the above publications.